1. Field of the Invention
The present invention relates to a fluid dynamic bearing. Particularly, the present invention relates to a fluid dynamic bearing which is suitable for a bearing used in an apparatus which requires nanometer order positioning accuracy such as an ultraprecision machine.
2. Description of the Related Art
In apparatuses which require nanometer order positioning accuracy such as an ultraprecision machine, a non-contact bearing such as an air bearing is used in order to eliminate the influence of friction occurred when a movable member is moved. In a fluid dynamic bearing such as an air bearing, a bearing gap therein is related to bearing rigidity, and when the bearing gap becomes small, the bearing rigidity becomes high. The gap of the bearing needs to be a few micrometers to cause high bearing rigidity and maintain a non-contact state of the bearing.
As a method of forming such a bearing gap in micrometer order, conventionally there was only a method of processing the individual parts configuring a bearing in dimensional accuracy of micrometer order or less, and assembling the bearing to obtain a few micrometer bearing gap.
It should be noted that, in a linear transfer machine in which an air bearing is used, in order to prevent the accuracy of motion from being reduced due to a change in the bearing gap which is caused by the heat generated from a coil of the linear motor, there is known a technology in which a cooling tube is inserted into the core around which the coil is wound in the linear motor to cool the linear motor so that the change in the bearing gap which is caused by the heat can be prevented (see Japanese Patent Application Laid-Open No. 1-194847).
In order to form a few micrometer bearing gap, in a conventional manufacturing method for a fluid dynamic bearing, the parts configuring the bearing need to be processed so that the few micrometer bearing gap can be obtained. In this case, since the dimensional accuracy of these parts is the micrometer order, the machining process for the parts becomes extremely difficult. Therefore, the dimension of the part has to be measured and processed repeatedly in order to obtain a bearing gap having a target size, and processing the parts by means of only the machining process requires considerable amount of time. Moreover, after processing the parts configuring the bearing, the bearing gap cannot be adjusted. If adjustment of the bearing gap is possible, a burden on the machining process of the parts configuring the bearing gap can be reduced. Such a fluid dynamic bearing in which the bearing gap can be adjusted does not exist in the prior art.
In the invention described in the abovementioned Japanese Patent Application Laid-Open No. 1-194847 as well is also designed to prevent a change in the bearing gap of the air bearing, and cannot adjust the bearing gap.